Fiberglass/polymer-matrix composites have seen increased industrial use due to their low density, versatility, and specific strength values. However, these composites consist of two highly dissimilar materials, which, at an interface between the fiber and matrix, form an interphase region. This interphase region typically is the weakest point in the composite due to insufficient bonding. To alleviate this problem, industrially, silane coupling agents have been applied via surface treatments to the fiber reinforcements. These agents have been shown to increase the strength and hygrothermal stability of the composites. In view of the increasing reliance on such polymer matrix composites, a need exists for an improved understanding of the chemical inter-reactions occurring within the interphase region, which now further comprises the silane coupling agents.
One technology that is particularly useful in monitoring and analyzing a fluid is spectrometric examination of light that passes through optical fibers disposed in the fluid, wherein the fibers act as evanescent wave sensors. An evanescent wave is electromagnetic radiation that results from the propagation of light through a light-conducting medium and that is present outside of the light-conducting medium. When light is transmitted through a high index of refraction medium (an optical fiber) the evanescent wave (or field) is produced in an adjacent lower index of refraction material and has intensity only within a fractional wavelength distance from the interface between the two mediums. Thus, spectrometric examination of light passed through optical fibers disposed in the fluid can reveal characteristics and properties associated with that fluid, at least immediately adjacent the optical fiber.
U.S. Pat. No. 5,712,934 to Johnson, for example, discloses an optical sensor using an optical fiber. However, the sensor operates on a principal requiring a return bend in the fiber, which leads to relatively complicated structures. Also, cladding that surrounds the optical fiber must be removed in the sensing regions to promote proper interaction between the optical fiber and the fluid being analyzed. Additionally, the fiber used by Johnson is not transparent in the infrared wavelength region and, accordingly, cannot be used with infrared spectroscopy, which is desirable in studying polymers, for example.
Similarly, U.S. Pat. No. 5,525,800 to Sanghera et al. and U.S. Pat. No. 5,585,634 to Stevenson et al. disclose optical fibers used as sensors wherein the cladding surrounding the fiber core must be removed in the sensing area. Further, according to Sanghera et al., a polymer is disposed in the region where the fiber has been stripped of its cladding, the polymer having a lower refractive index than that of the core of the fiber, and having an affinity for chemicals that may be of interest.
The present inventors have also previously researched the use of optical fibers as evanescent wave sensors. Particularly, optical fibers incorporated in polymer matrix composites have been studied. Studies were conducted with Polymicro Technology FIP 100/120/140 fibers consisting of a 100 .mu.m diameter fused silica core, a 10 .mu.m fluorine-doped fused silica cladding and a 10 .mu.m polyimide buffer. As in the prior art patents discussed above, the buffer and cladding were removed from a portion of the fibers to allow the fused silica core to be used as a model reinforcement as well as an evanescent wave sensor.
The exposed silica core was transparent over the 12,000 cm.sup.-1 to 4,000 cm.sup.-1 near infrared region. In the study using this sensor system the bulk curing of Epo-Tek 328 (available from Epoxy Technology, Inc., Billerica, Mass., and a diamine hardener was investigated in situ using a Fourier Transform Infrared (FT-IR) spectrometer. In a subsequent investigation, .gamma.-aminopropyltrimethoxy silane (.gamma.-APS, available from Sigma Chemical Co., St. Louis, Mo., coupling agent was adsorbed from aqueous solution onto the fibers, which were then immersed in Epo-Tech 328 in the absence of a curing agent. Heat was added to this system to promote reaction of the epoxy with the amino silane.
A band was present at 4925 cm.sup.-1 due to the stretching-bending combination of the --NH.sub.2 of the .gamma.-APS. This band was seen to decrease with time at elevated temperature when immersed in the epoxy. Such a decrease was expected as hydrogen atoms are abstracted from the --NH.sub.2 by reaction with the epoxy ring of the resin. A greater fraction of the --NH.sub.2 groups reacted when lower initial .gamma.-APS solution concentrations were used to adsorb the .gamma.-APS.
These findings are significant in that this system allows for direct monitoring of interaction between an epoxy resin and an aminosilane coupling agent adsorbed to silica fibers. However, this system does not adequately simulate a typical industrial composite for a number of reasons. First, the diameter of commonly used glass fibers is around 10 .mu.m, whereas the silica core of the FIP fibers is 100 .mu.m, an order of magnitude greater than the industrial fibers. Second, fiber size and flexibility are of concern because of possible industrial sensing applications. Relatively large fibers act to weaken the composite, as they are very brittle upon removal of the buffer and cladding.
Third, the composition of a typical industrial fiber (fiberglass) is approximately 55% SiO.sub.2, 16% CaO, 15% Al.sub.2 O.sub.3, 10% B.sub.2 O.sub.3, and 4% MgO, whereas the fused silica fibers are greater than 98% SiO.sub.2. The composition of the fiber is relevant because it has been shown previously that substrate effects can significantly influence composite properties. In this regard, see F. Garbassi, E. Occhielo, C. Bastioli and G. Romano, Journal of Colloid and Interface Science, 117, No. 1 (1987); D. J. Dawson and F. R. Jones, "The Role of Silane Treatment on the Retained Interlaminar Shear Strengths Aqueous Conditioned Glass Fiber Composites," in Controlled Interphases in Composite Materials, H. Ishida, Ed. (Elsevier Science Publishing Co., 1990), pp. 409-415; and T. H. Elmer, "Glass Surfaces," in Silyated Surfaces, D. E. Leyden and W. T. Collins, Eds. (Gordon and Breach Publishers, 1980), pp. 1-30. Significantly, it was shown that quartz exhibited much different bonding characteristics when exposed to a silane coupling agent than did glass and alumina and that alkali ions influence interphase properties. See, for example, Garbassi et al. and Dawson et al., cited above.
Optically, evanescent wave sensing requires the refractive index of the interphase adjacent the fiber to be less than the refractive index of the fiber. The previously studied FIP fibers have a refractive index of 1.4 (near-IR), whereas most epoxies have a refractive of at least 1.45. To alleviate this problem, model low refractive index epoxies such as fluorinated polymer must be used in combination with the FIP fiber system, thereby adding cost and limiting the ability of that system from analyzing different types of epoxies.